1. Field of the Invention
The invention relates to programmable frequency dividers, and in particular to programmable integer and fractional frequency dividers.
2. Description of the Related Art
Conventionally, a device performing frequency division of a high frequency signal, dividing a high input frequency to a desirable output frequency, is known as a programmable frequency divider or direct programmable frequency divider.
At present, programmable frequency dividers are accomplished by two approaches. The first approach is a frequency divider with a pulse/swallow counter. The frequency divider utilizes a dual-modulus divider, a programmable counter, and a swallow counter to provide the functionality of integer programmable frequency divider. The advantage is that the circuit design is adaptive to operational division range. The disadvantages are that only integer division is allowable, and that the dual-modulus divider is used in multi-mode or multi-band, thus, dissipating much power, and resulting in the frequency divider being unable to provide optimal power distribution to the three utilized circuits under a fixed method.
The second approach utilizes a multi-mode fractional frequency divider, for example, a divide-by-2/2.5/3/3.5 cell, coupled to a zipper frequency divider. The multi-mode frequency divider uses a state machine to control phase switching to provide divisors. The zipper frequency divider comprises multiple dual-modulus division cells coupled in series, each a divide-by-2/3 cell. The advantage is that the frequency divider is capable of fractional division. However, high frequency circuit design remains a challenge owing to the state machine utilized in the multi-modulus divider. The state machine utilized in the multi-modulus divider and other circuits are designed according to the fractions in uses, thus, the fraction choices are limited. Further, since the minimal divisor for the zipper frequency divider is 2N, N is the number of dual-modulus dividing cells, the divisors in the programmable frequency divider are limited, leading to a requirement of more dual-modulus dividing cells for the operational divisor range.